1. Field of the Invention
The present invention relates to focal plane array imaging devices. More particularly, the present invention relates to architecture and methods for employing a focal plane array for thermal imaging in a staring system.
2. Related Technology
Imaging devices in the form of focal plane array (FPA) configurations are well known in the art. These conventional FPA's traditionally consist of light-responsive sensors called picture elements ("pixels") arranged linearly or in an orthogonal, usually rectangular pattern of rectilinear rows and columns of photo-responsive detectors on the face of a semiconductor substrate. Hereinafter, terms such as "light" "radiant energy", and the like mean electromagnetic radiation including, but not limited to, visible light, invisible infrared or ultraviolet radiation, and other radiation.
Each pixel location may possess photocell circuitry designed to provide at least one electrical output signal that is directly related to the intensity of electromagnetic energy (visible light or infrared light, for example) impinging on that pixel location from a source object to be imaged. When these pixels are interrogated or sampled, usually in sequence within a respective row or column, the individual electrical output signals are accessed and supplied to external electrical processing circuitry. The electrical output signals may be digitized to provide a stream of data words carrying information relating to the intensity of incident energy at each pixel. The information may then be summarized or integrated to form an image representing the source object. This image may be presented, for example, on a display device such as a CRT (cathode ray tube), or LCD (liquid crystal display).
In a scanning imaging system (i.e., one wherein the source object or objects are not continuously viewed), the sampling frequency for the pixels may be varied by controlling the timing of the scanning or readout of the focal plane array. In contrast with staring systems, a scanning system uses a linear array of pixels across which successive image portions are swept by a moving scanning mirror. The sampling frequency of such a scanning system may be varied at the input to the array merely by changing the timing of the scan of the object to be imaged. The discussion herein will be confined to staring systems, which require no such mechanical scanning device.
In order to test the resolution of conventional focal plane arrays, a four bar arrangement of parallel bars each having a one-by-seven aspect ratio is employed as a standard test pattern. These bars have a definite temperature difference from the background scene. When the bar pattern is oriented vertically, a sampling of the pixels in a column-by-column interrogation (i.e., repeatably sampling of successive pixels in a column, and then successive pixels in a next adjacent column) provides resolution data in the horizontal or azimuthal direction of the test source. Similarly, vertical resolution is tested by use of a row-by-row sampling of horizontally oriented bars. Discussion herein will be limited to the measurement of azimuthal resolution.
For thermal imaging systems, a significant measure of detector performance is the MRT, or minimum resolvable temperature difference of the object in comparison to the background scene. This test is a measure of the signal energy at the detector surface of the pixels of a FPA. Up to a theoretical limit, known as the Nyquist limit (to be discussed below), the MRT for a given FPA may be plotted against the frequency of sampling of the pixels in the FPA to provide a graphical presentation of system performance. It will be recognized that the light flux integration period for particular pixels of an array varies as the reciprocal of sampling frequency. That is, as sampling frequency increases, the light flux integration interval for the pixels decreases. Thus, a frequency is reached at which the pixels cannot resolve a particular difference in temperature between the bar pattern and the background scene.
The frequency of pixel sampling may be expressed as the number of sampling incidents occurring for a particular arc subtended by the source object, e.g., in cycles per milliraidian. It is desirable to be able to measure the MRT difference of an array at the higher spatial frequencies where the detector resolution is high. Unfortunately, for all spatial frequencies above the theoretical Nyquist limit, MRT difference measurements are not able to be measured following current industry standards for conventional detector arrays, either staring or scanning.
In communication theory, the maximum time between regularly spaced instantaneous samples of a signal of bandwidth W for complete determination of the signal wave form is known as the Nyquist interval. This maximum time interval is derived to be 1/2W seconds. The reciprocal of the Nyquist interval, expressed as a frequency, is the Nyquist limit.
Heretofore, practitioners of the relevant technology have relied on a published and widely accepted model for relating the Nyquist limit to MRT difference measurement. According to that model, known as FLIR 92, MRT becomes indefinitely large at the Nyquist limit, so that attempts to measure performance at any higher spatial frequency are futile. That the Nyquist figure represents the operational frequency limit to meaningful MRT difference measurement is confirmed in the following pronouncement excerpted from the FLIR 92 document itself, as published by the U.S. Army Night Vision and Electro-Optics Directorate, Visionics/Modeling Division.
MRT difference is defined for a periodic target ( four 7:1 aspect ratio bars) , and the criterion for "calling" MRT difference at some frequency is that the four bars must be fully resolved by the observer. In thermal images, the four bars of the MRT difference target will never be fully reconstructed to the observer at frequencies beyond a system's Nyquist limit, and therefore, the criterion for calling MRT difference cannot be met. FLIR 92 adheres strictly to this definition by not predicting MRT differences at frequencies beyond the Nyquist limit.
As the most current version of an earlier (FLIR 90) model, FLIR 92 thus presents, as the imaging industry standard doctrine, that ". . . because the ability of observers to interpret information for target discrimination at Super Nyquist frequencies (i.e., above the Nyquist limit) has not been quantified, attempting to extrapolate without robust data is unacceptable".